1. Field of the Invention
This invention relates to an oxygen concentration sensor. More specifically, the present invention relates to an oxygen concentration sensor suitable for detecting an oxygen concentration in an exhaust gas of an internal combustion engine, for example.
2. Description of the Related Art
To produce an oxygen concentration sensor for detecting an oxygen concentration in an exhaust gas emitted from an internal combustion engine for an automobile, for example, methods of forming electrodes by non-electrolytic plating, vacuum deposition, screen printing, and so forth, have been known in the past.
Among them, the oxygen concentration sensors having the electrodes formed by non-electrolytic plating, vacuum deposition, etc., have the advantage that since the thickness of the electrodes is relatively small, the sensors can quickly cope with the change of the oxygen concentration in a gas to be measured and can detect the oxygen concentration.
In the oxygen concentration sensors having the electrodes formed by screen printing, on the other hand, screen printing is effected on a green sheet using a platinum paste, or the like, and a plurality of green sheets are simultaneously sintered. Accordingly, the thickness of the electrode becomes relatively great. In other words, the thickness of the electrode formed by screen printing is by 5 to 20 times greater than the thickness of the electrode formed by non-electrolytic plating, vacuum deposition, etc., and for this reason, the oxygen concentration sensors having the electrode formed by screen printing have high heat resistance.
However, since the electrode formed by screen printing has a relatively large thickness in the oxygen concentration sensor having such an electrode, a gas diffusion distance from the electrode surface to a three-phase boundary point (triple point) becomes great. Accordingly, there occurs the problem that diffusibility of the gas to be measured in the electrode is deteriorated and the response speed of the sensor drops, particularly remarkably at a low temperature.
To solve this problem, a method is known in the art in which ceramic powder, etc., are mixed into the platinum paste, to restrict sintering of the platinum particles during sintering, to thereby obtain a high porosity of the electrode. According to this method, however, the effect of the large thickness of the electrode is still great and the effect of improving the response is not so extreme.
SUMMARY OF THE INVENTION
The present invention was completed so as to solve the problem with the prior art described above, and is directed to providing an oxygen concentration sensor which not only is excellent in heat resistance and durability but also maintains a high response.
To accomplish the object described above, according to the construction of the first embodiment of the present invention, the measurement electrode is formed by two kinds of electrodes, that is, the first electrode and the second electrode. In the first embodiment of the present invention, when the first electrode is particularly formed as the skeletal region and the second electrode is particularly formed as the reaction region, the thickness of the first electrode is made greater than that of the second electrode and the porosity of the former is lower than that of the latter.
Accordingly, since the first electrode is so formed as to have a greater thickness than the second electrode, the first electrode has higher heat resistance than the second electrode. Nonetheless, because the second electrode is so formed as to have a relatively small film thickness but to have a greater porosity than the first electrode, the second electrode can secure higher response than the first electrode.
According to the construction of the second embodiment of the present invention, the second electrode has a smaller film thickness but higher response than the first electrode. Though the second electrode has lower heat resistance than the first electrode because its thickness is smaller than that of the latter, a higher response can be obtained.
In the constructions of the first and second embodiments of the present inventions, the combination of the first electrode with the second electrode can simultaneously realize both high heat resistance and high response, which are generally believed to be contradictory to each other.
As explained above, in the oxygen concentration sensor according to the present invention, the measurement electrode to be formed on one of the main surfaces of the solid electrolyte comprises the heat-resistant thick first electrode for primarily forming the skeletal region and the high response thin second electrode for primarily forming the reaction region, in the composite structure. Accordingly, the oxygen concentration sensor provides an excellent effect in that excellent heat resistance of the first electrode and excellent response of the second electrode can be fully exploited in combination.
These and other objects and novel features of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings.